Portable navigation devices (PNDs) that include GPS (Global Positioning System) signal reception and processing functionality are well known and are widely employed as in-car or other vehicle navigation systems. PNDs of this type may typically be mounted on the dashboard or windscreen of a vehicle, but may also be formed as part of an on-board computer of the vehicle radio or indeed as part of the control system of the vehicle itself. Conventional PNDs have an operation predicated on “forwards driving” in which the heading, i.e. orientation, and measured movement direction of a vehicle are substantially aligned. “Backwards driving”, i.e. reverse driving, in which a vehicle is travelling in reverse, typically along a road segment, is not specifically addressed in such navigation devices. One of the main features of a navigation device is to display, in substantially real-time, the movements, e.g. position and heading, of the device (or vehicle, if the device is associated with a vehicle) on a representation of an electronic map, e.g. a graphical representation of the road network. This functionality requires two underlying processes: map “matching” to project a latest received position onto a corresponding position on a navigable segment of the electronic map; and “prediction” to smooth the transition between consecutive received positions.
Applying the “matching” and “prediction” processes of a conventional navigation device (which is configured only for “forward driving” operation) gives rise to complications and potential distractions for the driver. Thus the prediction process and matching process may “cooperate” to represent a reverse driving vehicle movement as oscillating, e.g. appearing to perform one or more sudden U-turn.
There is therefore a need to be able to distinguish between situations where the vehicle is driving in reverse and those where it has performed a genuine U-turn. The data necessary for making such a distinction can be obtained from vehicle sensors. Vehicle sensor data may be conveniently available to navigation devices integrated within the vehicle. Alternative sources of data necessary for identifying reverse driving are available and some of these may be used to detect backwards movement with portable devices, e.g. by processing camera images.
Although some of the problems associated with backwards driving could be solved by maintaining both forwards and backwards road graph expansions at the same time, this is typically not possible given the memory and processing resources available on conventional navigation devices. There is therefore a need for a method for handling “backwards driving”, in a more processor-efficient manner.